The present invention relates to a communication system for use in an area having a human environment delimited from below by a floor and from above by an overhead environment substantially clear of human presence.
More particularly, the present invention relates to a communication system comprising one or more first transmitter/receiver units, called "controllers", arranged in an overhead environment, for producing a substantially downwardly directed communication beam, and a plurality of second transmitter/receiver units, called "communicators", arranged within the human environment.
A communication system of this type has many uses and applications; for this reason the communication system may be termed a "universal communication system". One of the principle applications of the invention, however, is to provide communication between hand-held or desktop computers in a so-called "wireless local area network" or "wireless LAN".
In order to avoid the necessity of completely rewiring a building, and in order to provide a communication link to mobile devices, such as notebook computers which are moved from place to place, conventional, hard-wired LAN's are being replaced by the more convenient wireless LAN. The design goals for a wireless LAN communication system include the following:
(1) Wide Band: In order to accommodate the vast quantities of information that must be transmitted back and forth between terminals, it is desirable that the communication link have many megabytes per second of bandwidth. PA1 (2) Low Power: Since hand-held terminals in particular must operate on battery power, it is desirable that the power requirements for transmitting from such terminals be extremely low. PA1 (3) Low Radiation Hazard: It is desirable to keep the radiation levels to a minimum to avoid health hazards to people in the LAN environment. It is well known that high levels of radiation can cause cataracts, cancer and even brain damage. PA1 (1) a controller mounted in an overhead environment for illumination downward within a cone shaped region; and PA1 (2) a communicator which is located within the human environment; i.e., in the region of the floor up to a height of about two meters (six and a half feet). PA1 (1) an oscillator for generating a carrier signal at substantially the same frequency as that of the downwardly directed communication beam(s) produced by the controller(s); PA1 (2) a device for modulating (preferably amplitude modulating) the carrier signal with an independent information signal to produce a modulated carrier signal; PA1 (3) a device, including an amplifier and an antenna, for actively transmitting the modulated carrier signal within the human environment; and PA1 (4) a control device for controlling the operation of the system in such as way as to enable the transmission of the modulated carrier signal only during periods in which normal wireless LAN communication is not taking place.
One type of wireless LAN that has been proposed includes a central radio base station which communicates with all the terminals in an entire building or floor. In order to overcome the effect of path loss, both the base station and the terminals must transmit with considerable RF power. In addition, in order to cover a large area with a single radio source and limit the effects of reflections, it is necessary to limit the bandwidth of each separate communication.
Another well-known wireless LAN provides an ad hoc communication link between terminals in the same room. Communication is effected by transmitting an infrared signal which may be received almost everywhere within the room or partitioned area either by direct line of sight or after reflection from the walls and/or ceiling. Although infrared transmission meets the bandwidth, power and radiation hazard requirements mentioned above, the signal cannot pass through walls, be they permanent or merely temporary barriers, and the infrared link is not tolerant of the sunlight which may enter through windows of the building.
The U.S. patent application Ser. No. 07/477,996, filed Apr. 16, 1990, entitled "Passive Universal Communicator System", now U.S. Pat. No. 5,164,985, discloses a LAN which provides wireless communication between computer terminals, telephones, video terminals and the like, at a local site (e.g., within a building).
This communication system, which avoids many of the difficulties of the prior art, comprises essentially two types of elements:
The controller transmits a communication beam, such as a microwave beam, and imparts "first information" to this beam. The communicator, in turn, reradiates this beam back toward the controller and imparts "second information" to the reradiated beam.
The "first information" and "second information" are completely independent of each other. For example, the "first information" can be video information intended to be displayed on the communicator's video screen. The "second information", for example, may be typewritten text entered into the keyboard of the communicator by the terminal user.
The communicator has no transmitted energy power source of its own. It simply receives the incoming beam from the controller, detects the "first information" on this beam, modulates the beam carrier with the "second information" and reradiates this same beam toward the controller for receipt and demodulation by the controller. The reradiated beam containing the "second information" is known in the art as "modulated backscatter".
While this system fulfills all the design requirements noted above and is extremely robust in its operation (in the sense that it cannot be effected by sunlight or other environmental influences), it has a disadvantage that it is unable to form an ad hoc network; that is, a network formed among a few communicator terminals within a small region of space. Instead, for a communication between one communicator and another, either within the same room or outside the room, it is necessary for a first communicator to send its "second information" to the controller immediately above. This controller is in communication with all other controllers within the LAN; these controllers, in turn, transmit the "second information" received from the first communicator. All other communicators within the network are thus capable of receiving this information.
While this mode of communication is safe and reliable, it is unnecessarily cumbersome for certain circumstances. For example, if is desired for one computer terminal to transmit to a single adjacent computer terminal, the extensive capability of the wireless LAN is not required.